The present invention relates to a method for clearly and assuredly printing the surface of a bag which has been filled with a content composed of powder or granule with the grade or number of the content, and to the apparatus for effecting the same.
In most cases, it is required to print additionaly the surface of a bag which has been filled with a content composed of powder or granule with the grade, factory name, date and production number.
In cases where the surface of the bag which has been filled with such a content is printed, if the content filled in such a bag is a highly flowable material such as powder or granule, the content in the bag is caused to flow in the direction indicated by the arrow in FIG. 1 by the pressure exerted by a printing plate, with the result that the surface of the bag is deformed. As a consequence of such deformation, certain portions of the bag are not subjected to the pressure produced by said printing plate, resulting in unclear printing. Thus the prior art method has a great defect.
As a result of intensive investigations made for the purpose of obviating such a defect that the surface of a bag which has been filled with a content composed of powder or granule is not clearly printed according to the prior art method, it has been found that the surface of the bag can clearly be printed without causing the content to flow by the pressured produced by a printing plate by first removing the air in the bag by pressing to flatten the surface of the bag, supporting and transporting the flattened bag in such a manner that it assumes an arc shape having a fixed curvature with which the convex surface of the bag is permitted to be opposed to the convex surface of the printing plate and allowing the surface of the bag to be clearly printed by the printing plate in a state where the surface of the bag is tensed at a position just below the printing plate while pressing the content.